1. Field of the Invention
This invention relates to transponder systems, and more particularly to communications transponders such as satellites which receive input signals at multiple channel frequencies, process the received signals and retransmit them at other frequencies.
2. Description of the Related Art
Satellite transponders typically receive ground-generated signals within one frequency band, such as the K.sub.u band (12.5-18 GHz), process the received signals, and then retransmit them back to earth within another frequency band such as the X-band (8-12.5 GHz). The transponder includes numerous channels that are allocated to communication traffic, with the channels separated from each other in frequency. For example, a commercial satellite will typically have 24 channels dedicated to different customers.
The signals received by the satellite are processed, typically by a gain control amplifier circuit, before being retransmitted. Each channel may have different processing requirements, since each customer may have unique applications. Thus, separate gain control amplifiers are provided for each channel.
The common approach currently used for satellite communications is to convert the frequency of each received signal down to its corresponding frequency for transmission back from the satellite, and to perform the gain control processing at the transmit frequency. Filtering is also performed at the transmit frequency to eliminate spurious signals. This type of system is described in Pritchard and Sciulli, Satellite Communication Systems Engineering, Prentice-Hall, Inc., 1986, pages 282-284. Performing channel filtering and gain control at the relatively high transmission frequencies requires expensive and complex microwave components to operate properly within a frequency range that may be as wide as 4-14 GHz, with a bandwidth of 500-000 MHz. Such wide bandwidths at the transmission frequencies make it very expensive and time consuming to tune acceptable signal responses over changing temperatures and frequencies. Furthermore, discrete or at least multichip components must be used to handle the relatively high frequencies, making it very difficult to implement any of the primary functions with a single chip integrated circuit.
The present approach also requires custom designed input multiplexers for each communications channel. In addition to adding to the cost and complexity of the transponder, the need for custom designed elements delays the design cycle. A satellite's frequency plan must be coordinated with all other possible sources of interference, both on the ground and in space. Because of the customized nature of present transponder architectures, satellite design and manufacturing cannot begin until permission from the Federal Communications Commission has been granted. Since the input multiplexers are custom designed, each satellite requires a mechanical design team to lay out the multiplexers and a receiver network to provide the proper interconnections and redundancy switching; this is a highly complex and expensive operation. Furthermore, switching is performed with heavy mechanical switches because of their physical location in the layout and their reliability.
It would also be highly desirable to have a "frequency nimble" transponder, meaning one that is capable of easily adjusting its channel frequencies. However, the customized aspect of present transponder designs, with all filters set to fixed frequencies, does not allow for changing frequencies. For example, if a channel user finds a different market application that requires a different frequency scheme after the transponder has been designed, the customer is locked into the original frequency scheme unless it invests the substantial amount of money and time necessary for a new transponder design. Once in-orbit, no changes to the frequency scheme can ever be made.
A known technique that does provide a degree of frequency nimbleness converts all of the received signals down to lower frequencies by a common down conversion factor, processes the channel signals at their separate lower frequencies, and then up converts the signals for transmission. Such an approach is described in Hughes Aircraft Company, Geosynchronous Spacecraft Case Histories, Volume II, 1981, pages iii and 2-7a.3 through 2-7a.7. However, it again requires a high degree of custom designed equipment that adds significantly to the system's cost and complexity.